1. Field of the Invention
The present invention relates to Doherty amplifiers, and, more specifically, to the integration of a quarter wave transformer/combiner circuit with a Doherty amplifier.
2. Related Art
The Doherty amplifier, developed in the 1930""s, is a linear power amplifier that operates at low power consumption levels and high efficiencies. For an overview of the Doherty amplifier, see Doherty, W. H., A New High Efficiency Power Amplifier For Modulated Waves, Proceedings of the Institute of Radio Engineers, Vol. 24, No.9, pp. 1163-82, September 1936, which is hereby incorporated by reference.
Although the Doherty amplifier has advantages in certain applications, problems have been encountered with applying the Doherty amplifier to low cost radio frequency integrated circuits (RFIC) for use in wireless communications equipment. In the past, Doherty amplifiers were designed at low enough frequencies and/or high enough impedances that parasitics such as bond-wire inductance and stray capacitance were not a significant factor. As such, topologies emerged for these applications where portions of the amplifier system were implemented off-chip, that is, outside the RFIC containing the Doherty amplifier, and downstream from any impedance matching circuit.
A problem with these topologies is that parasitic elements such as bondwires, stray capacitance, and losses in the matching circuit, which become more prominent at the high frequencies inherent in RF applications, can degrade the performance of the Doherty amplifier. Moreover, any phase shift that occurs in the matching circuit can further degrade the performance of the Doherty amplifier.
The present invention provides a Doherty amplifier system where a quarter wave transformer/combiner circuit (QWTCC) is directly coupled to the outputs of the carrier and peaking amplifiers of a Doherty amplifier. The QWTCC achieves about a relative net 90xc2x0 phase shift between the carrier and peaking amplifier outputs so that the amplifier outputs, after the phase shifting, are approximately in phase. The QWTCC then combines the relative phase-shifted outputs together at one or more nodes. An impedance matching circuit may be coupled between the one or more nodes and a system output in order to achieve a desired impedance at the system output. The QWTCC together with the Doherty amplifier may be implemented on one RF integrated circuit chip.
The QWTCC may be implemented as a plurality of circuit elements in a xe2x80x9cpixe2x80x9d network. In one example, the QWTCC includes an integral number of pi sections coupled in parallel, each section comprising a series combination of a shunt inductance (L), series capacitance (C), and shunt inductance (L). Each section may be coupled in parallel between the outputs of the carrier and peaking amplifiers. Each of the pi sections may also be coupled to a node, and the nodes for all the sections may form one or more outputs of the QWTCC. An impedance matching circuit may then be coupled between the one or more QWTCC outputs and a system output.
The shunt inductance elements in this example may be configured to provide collector or drain bias to the carrier and peaking amplifiers. These shunt inductors may be realized using bond wires that are generally used to connect an integrated circuit to its external circuitry. The series capacitance elements may be configured to provide DC isolation between the amplifiers and may be integrated directly on the RFIC.
The load impedance presented by the QWTCC to the carrier amplifier in this example may decrease as the input power increases. This allows the efficiency of the system to be held relatively constant over a selected power range.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with still in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.